24 January 2012

Is it a fish, is it a bird? ...It’s an eagle ray!

They fly like birds under water and create strange pits in the sand. Eagle rays can be seen around New Zealand’s coast in the summer months, when they come in to breed. Like their larger cousins, the longtail and shorttail stingrays, they have a sting in their tail.

Eagle rays appear to fly through the water, gracefully beating their large pointed pectoral fins like wings. “This distinguishes them from stingrays, which undulate through the water,” says NIWA fisheries scientist Bruce Hartill. Their bodies are wider than they are long – up to 1.5m across, with females larger than males – and tails longer than their bodies. Eagle rays get their name from their protruding heads, which appear eagle-like in profile.

“While eagle rays are not aggressive, they can deliver a painful sting with their tail, says NIWA fisheries biologist Malcolm Francis. “This is likely to contain a protein-based venom, as immersing the injured area in hot water appears to neutralise the pain.”

Eagle rays are found in coastal waters and estuaries around New Zealand and the Southwest Pacific, spending most of their time in shallow waters down to depths of 160 m. They’re occasionally seen as far south as Foveaux Strait, but are mostly found around the North Island, and as far north as the Kermadec Islands. They spend most of their time over sandy and muddy bottoms, but are seen occasionally around reefs.

Eagle rays are generally solitary – more so than stingrays – but will congregate in shallow water during the summer months, when they come in to breed.

“Females come into bays and estuaries in early spring to give birth to live young,” says Mr Hartill. “The young are perfectly formed miniatures of the adults, less than a foot across.

“Once the females have pupped, the smaller males come in to mate in January/February. The males grip onto the back of the female with their plate-like teeth, leaving telltale round scars. They fertilise the female using ‘claspers’ that protrude from the base of their pelvic fins.

“The males head back out to deeper water before the females, who stay near the coast a bit longer to fatten up after pupping.”

Eagle rays belong to the group of cartilaginous fish – which includes sharks and chimaeras – whose skeletons are made of cartilage rather than bone. They can detect their prey even when they can’t see them, using a well-honed electric sense: jelly-filled pores on their head can detect the weak electrical fields created by the muscles of other animals.

Eagle rays use these electro-sensory organs to find shellfish and other prey buried in sand or mud. “They blast a jet of water out of their gills to excavate the sand around their prey, leaving telltale pits behind,” explains Mr Hartill. “You can often see these pits in bays and estuaries at low tide in the summer months – they’re about a foot across, with steep sides.”

Their prey – which includes scallops, oysters, worms, and hermit crabs – are crushed by strong, broad plates of teeth. They consume only the meat, leaving piles of broken shell behind.

Eagle rays are not fished commercially, but are sometimes accidentally caught. “Fishermen tend to cut off their tails to avoid a sting, and because they get tangled in nets,” says Mr Hartill. “This doesn’t kill them, but leaves them defenseless against their main predators: orcas.”

Species Fact file

• Common names: New Zealand eagle ray
• Māori name: Whai keo
• Scientific name: Myliobatis tenuicaudatus
• Type: Cartilaginous fish
• Family: Myliobatidae – Eagle rays
• Distribution: Temperate waters of Southwest Pacific: Norfolk Island and New Zealand, including Kermadecs.
• Habitat: Bays, estuaries and near rocky reefs in 0-160m depth range.
• Size: Max width: 150cm; wider than they are long.
• Lifespan: Females: up to 25 years; males: up to 15 years – data from counting rings on backbone.
• Diet: Scallops, gastropods, oysters, worms, crabs.
• Reproduction: Ovoviviparous – gives birth to live young.
• Things you need to know: If stung by the spine on the eagle ray’s tail, the poison is quickly neutralised by immersing the injured areas in hot water.

Something strange: Eagle rays uncover their shellfish prey from sand or mud by jetting columns of water out of their gills, leaving telltale excavation holes in the sediment.

ENDS

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